N-Acetylneuraminic acid (hereinafter, referred to as NeuAc) is a kind of acidic amino sugar generically called sialic acid and is widely used as a raw material of pharmaceuticals such as an anti-influenza drug, or as a component of food, cosmetics and cell culture media.
NeuAc can be produced by a fermentation method, an enzyme method, an extraction method from a natural product, a chemical synthesis method, or the like. As the known NeuAc crystal, crystals of a dihydrate (Non-Patent Document 1) and an anhydrate (Non-Patent Document 2) are known, but a solution of such a crystal shows strong acidity, and the degree of solubility thereof is relatively low of about 100 g/L at normal temperature. Furthermore, there is a problem with storage stability, for example, the crystal and a solution thereof are gradually discolored to dark brown.
On the other hand, as to the crystal of an ammonium salt of NeuAc, a crystal of monohydrate is known (Patent Document 1). This crystal has a problem with stability under high temperature conditions because of its low melting point near 105° C.
Under such circumstances, a crystal of an ammonium salt of NeuAc, having high storage stability at normal temperature as well as under high temperature conditions, is demanded.